The invention relates to a procedure for measuring angles and angular characteristics by means of a gyro.
Measuring angles and especially angular characteristics, i.e. the interdependence between angular deformations or displacements of structural parts and acting forces or torques can be considered as a significant problem in the technical field, a problem which cannot easily be solved.
In automotive engineering deformations of the body have to be measured as a function of exterior loads. In civil engineering this is the case with angles and deformations of bridges and buildings. Also in aircraft industry the interdependence between the deformations of the fuselage and wings and the acting forces or torques has to be measured, before new models go into production. In windtunnels aerodynamic measurements with aircraft models are carried out, whereby forces and torques are measured as a function of displacement angles. Finally, in geodesy angles between reference directions have to be measured.
For the precise measurement of angles and angular characteristics measurement procedures are often being used which are difficult to handle and susceptible to trouble.
For many applications angle encoders which are advantageously used with machine tools and robots, cannot be used, as their measuring axes have to coincide with the respective accurately defined axes of rotation.
Optical measurement procedures like autocollimation require a fixed basis such as a tripod on a foundation. Also a mirror has to be fixed on the deforming part and it has to be adjusted into the optical path which it tends to leave under acting load.
Inclinometers which, in principle, are accelerometers, are easier to handle in this respect. Neither do they require a fixed basis nor a well defined axis of rotation. They measure, however, only angles with respect to the vertical and are susceptible to horizontal accelerations. This restriction often is of disadvantage.
Gyros measure angular rates with respect to an inertially fixed direction which, as it is generally known, can be defined in any direction. Due to the rotation of the earth with respect to this inertially fixed direction, the corresponding component of earth rate has to be known, if the measurement of the angular rate with respect to the earth is considered. Only after compensation of earth rate and measurement errors, i.e. gyro drift and scalefactor errors, the gyro measures the angular rates with respect to the earth with high precision, which allows also to compute angles in a computer via integration. Under these conditions gyros are easy to handle angular sensors.
In inertial navigation systems (INS) which are part of any modern aircraft, attitude and heading angles of the aircraft with respect to earth-fixed directions are obtained by means of three gyros, three accelerometers and a digital computer. But for the mere measurement of angles and angular characteristics an INS is much too costly. Besides, the sensor errors in an INS would have to be corrected for a precise implementation of this measuring task, which, at the present state of the art, is done through the use of calibration and aiding procedures.
Calibration procedures for the determination of the sensor errors are carried out before or after the measurement process, as known external references are then available, They require however a lot of time as the gyro drift is computed by means of an integration process over a period of time--its accuracy increasing with calibration time. On the other hand the achievable accuracy for measuring angles and angular characteristics is limited, if sensors are used whose drift parameters are varying with time, as the calibration procedures mentioned are based on a certain stability of these parameters in the course of the measurement procedure.
There is further known (B. Stieler, H. Winter "Gyroseopic Instruments and their Application to Flight Testing", AGARD-AG-160-VOL.15, 1982) the application of so-called aiding procedures requires the processing of known external reference data during the measurement procedure while using special mathematical algorithms (e.g. Kalman filtering). In comparison with the above-mentioned calibration procedures they can still successfully and to a larger extent be used when sensors of minor quality, i.e. with instable drift parameters are employed. The disadvantage in the application of this procedure lies, of course, in the setup of such external measurement data, which is often altogether impossible. The invention is however also a valuable supplement for the known aiding procedures.
A procedure of this kind is known for the measurement of pipelines, with the aid of the so-called pigs, which are taken along by the medium transported as separating elements between the individual charges (U.S. Pat. No. 4,799,391). Like an aircraft these pigs are equipped with an INS, with three gyros, three accelerometers and a digital computer. For the computation of the pipeline's curvature as a function of the pig's position in the pipeline the system is in addition equipped with a time basis. For the definition of position by means of an INS external measurements for INS aiding are employed such as indicators along the, pipeline, for instance magnetic anomalies in form of girth welds or similar markers.